This invention relates to an integrated circuit for detecting changes in light intensity and more particularly to such an integrated circuit having a noise immune voltage regulator supplying the back bias voltage on a photo-diode.
Intrusion alarms, proximity sensors, and other systems, are known, which systems operate on the principle that a signal is produced when a body moves between a photo-sensor and a fixed source of light causing a reduction or an increase in the absolute intensity of the sensed light. Camera photo-sensors likewise produce a signal that is related to the absolute light level being sensed. Unlike the above noted light sensing systems, it is a change in light level that is of interest here and not the absolute level of light which is to be detected.
A conventional means for light detection is comprised of a photo-diode that produces a current which is proportional to the light being sensed at any instant of time. The photo-diode is typically connected so as to be back-biased between a d.c. voltage buss and the input of a current amplifier. An integrated circuit of this kind is disclosed by Genesi in patent application Ser. No. 677,573 filed Apr. 16, 1976, now U.S. Pat. No. 4,085,411 issued Apr. 18, 1978 and assigned to the same assignee as the present invention.
Fluctuations or noise voltages appearing superimposed on the d.c. voltage supply buss tend to be transmitted directly to the input of the photo-amplifier through the inherent PN junction capacity of the photo-diode. In a photo sensing system responsive only to a change in light level, such noise signals would tend to produce a seriously distorted and false output signal.
This problem is exacerbated when it is additionally required that the sensing system be battery operated and/or have a self-contained relatively high power alarm-signal producing means operable from the same battery power supply. Such alarm-signal means such as flashing lamps, buzzers, etc., typically generate large amplitude noise signals on the aforementioned power buss, and dry battery power sources typically do not provide a low impedance source that would tend to minimize this noise. Noise may also be induced in the power buss by nearby motors, relays or other power handling equipment.
A conventional solution to this kind of problem consists in providing power buss noise isolation filters typically consisting of an impedance inserted in series with the d.c. power buss at the noise sensitive photo sensing circuit plus one or more filter capacitors. Besides the extra expense of such filters, they typically reduce the d.c. voltage supplied to the noise sensitive circuits requiring in turn an increase in the battery voltage which leads to additional expense.
It is an object of this invention to provide a photo sensing integrated circuit that is responsive only to changes in light intensity and that includes an integrated supply voltage regulator for isolating the photo sensor from noise that may be superimposed on the d.c. supply voltage.
It is a further object of this invention to provide such an integrated circuit that is operable from a relatively low voltage d.c. source (e.g. 2.5 volts).
It is a further object of this invention to provide such an integrated circuit including an integrated high power alarm generating means which entire integrated circuit is powered from a common d.c. voltage source.